FIG. 9 is a side elevational view showing a conventional hydraulic elevator disclosed in, for example, Japanese Patent Application Laid-Open No. 7-20827 and FIG. 10 is a plan view showing the hydraulic elevator shown in FIG. 9. In these drawings, a pair of guide rails 2 are disposed spaced from each other within a hoistway 1. A car 3 that is raised and lowered along the guide rails 2 is disposed between the guide rails 2. A plurality of guide shoes 4 are fixed to the car 3 as guide members which engage with the guide rails 2, respectively.
First and second hydraulic jacks 5 and 6 are installed on one side of the car 3 within the hoistway 1. These hydraulic jacks 5 and 6 have jack bases 5a and 6a fixed on a floor 1b of a pit 1a of the hoistway 1, cylinders 5b and 6b fixed on the jack bases 5a and 6b, and plungers 5c and 6c that are reciprocated in a vertical direction inserted into the cylinders 5b and 6b.
Upper end portions of the plungers 5c and 6c are connected to each other by a support frame 7. A rotatable suspension sheave 8 is mounted on the support frame 7. A pair of guide rails 9a and 9b for guiding the ascending/descending movement of the support frame 7 are fixed to the upper end portions of the cylinders 5b and 6b, respectively.
A hydraulic power unit 12 having an oil tank 11 is installed within a machine room 10 adjacent to the hoistway 1. The hydraulic power unit 12 is connected to the first and second hydraulic jacks 5 and 6 through a pipe 13 branched at a midway portion thereof.
A stationary member 14 is fixed to the floor 1b of the pit 1b. The car 3 is suspended within the hoistway 1 by a suspension rope 15 wound around the suspension sheave 8. The suspension rope 15 has a hoistway side fastening end 15a fastened to the stationary member 14 and a car side fastening end 15b fastened to the car 3. The car side fastening end 15b is fastened to a rope support beam 17 of the car 3 through a spring 16.
The operation will now be described. A pressurized oil is fed into the cylinders 5b and 6b of the first and second hydraulic jacks 5 and 6 from the hydraulic power unit 12 so that the plungers 5c and 6c are raised and the car 3 is raised along the guide rails 2. In this case, the elevating velocity of the car 3 is twice as fast as the elevating velocity of the plungers 5c and 6c. Also, the hydraulic oil within the cylinders 5b and 6b is fed back to the hydraulic power unit 12 by the weight of the car 3, allowing the car 3 to descend.
In this case, it should be noted that there is a predetermined upper limit of working pressure for respective hydraulic equipment such as the hydraulic pump (not shown), the hydraulic valve (not shown), the pipe 13, the hydraulic jacks 5 and 6 and the like within the hydraulic power unit 12. The upper limit of the working pressure is set to be greater than a value obtained by dividing twice the sum of the respective weights such as the tare weight (dead weight) of the car 3, the load, the weight of the suspension sheave 8 and the like by the cross-sectional area of the plungers 5c and 6c.
In the thus constructed conventional hydraulic elevator, a coupling force (the pair of forces that are the same in magnitude, but opposite in direction) about a fixed point of the suspension rope 15 in the car 3 is received by the guide shoes 4. The load (guide shoe reactive force) applied to the guide shoes 4 is given by the equation: F=Wc.times.Ex/H, where F is the guide shoe reactive force, Wc is the weight of the car 3, Ex is the distance, in the opening direction of the doors, from the gravitational center of the car 3 to the suspension point thereof, and H is the interval between the upper and lower guide shoes 4 of the car 3, as shown in FIG. 11.
As described in the equation, the guide shoe reactive force is proportional to the distance Ex from the gravitational center of the car 3 to the suspension point in the opening direction of the doors. Accordingly, in a hydraulic elevator having a larger dimension in the opening direction of the doors of the car 3, there are certain instances where the car 3 can not be guided by the guide shoes 4.
For example, the guide shoe reactive force in a hydraulic elevator (car dimensions: opening direction of the doors dimension of 1,400 mm.times.depth dimension of 1,350 mm.times.door opening height of 2,100 mm) with a load of 750 kg, 11 persons standardized in accordance with Japanese Elevator Association is given as follows:
Namely, since the weight of the car is about 1.2 times the load, the relationship, Wc=750.times.1.2=900 kg, is established. Normally, the position of the center of gravity is substantially at the center of the car in the opening direction of the doors, and the dimension from an end of the car in the opening direction of the doors to the suspension point is about 150 mm. Accordingly, the distance from the gravitational center of the car to the suspension point in the opening direction of the doors is Ex=1,400/2+150=850 mm. Also, the interval H between the upper and lower guide shoes is normally about 3,000 mm.
If such conditions are substituted in the above-described equation, the relationship of the guide shoe reactive force, F=900.times.850/3,000=255 kg, is established. When the guide shoe reactive force becomes large, the cost is increased since it is necessary to enlarge the size of members such as the guide shoes, the guide rails and the car frame which are subjected to the guide shoe reactive force, resulting in a less economical elevator.
Also, in the above-described hydraulic elevator, since the fastening force of the plungers 5c and 6c on the packing (not shown) for preventing oil leakage from the sliding portions between the cylinders 5b and 6b and the plungers 5c and 6c, respectively, varies, the travel resistance of the plungers 5c and 6c also fluctuates. Accordingly, even if the hydraulic jacks 5 and 6 are controlled in the same manner, the extension speeds of the plungers 5c and 6c become different from each other, resulting in abnormal wear of the packing due to the slant of the plungers 5c and 6c and the application of an overly large force to the support frame 7. These factors cause the hydraulic elevator to breakdown.
Also, Japanese Patent Application Laid-Open No. 62-264186 and Japanese Patent Application Laid-Open No. 8-268664 show a hydraulic elevator in which a car is raised and lowered by using a plurality of hydraulic jacks. However, since there is a single suspension point for the suspension rope, a large reactive force is applied to the guide members for guiding the movement of the car in the vertical direction.